1. Field of the Invention
The present invention relates to a resinous composition for denture base which is particularly suitable for use in the polymerization method of dentures by irradiation of microwaves (hereinafter called the microwave curing method), but is applicable to the usual polymerization method thereof by heating in water bath.
According to the microwave curing method, a dough-form mixture of resinous compositions for denture base comprising a powdery component and a liquid component is filled and pressurized in a plaster denture negative model previously formed in a glass fiber-reinforced plastic flask (hereinafter simply referred to as the FRP flask), and is then irradiated with microwaves for polymerization. The compositions according to the present invention may be polymerized by the ordinary heat curing method. Especially when the microwave curing method is applied, however, they can be polymerized in a short period of time by irradiation of microwaves, thereby yielding a set product that is homogeneous and free from any porosity, and making it possible to prepare a denture of improved fitting accuracy.
2. Statement of the Prior Art
According to the the ordinary heat curing method for preparing dentures, a previously prepared wax denture is invested in plaster with the use of metallic flask. After the investing plaster is set, the wax is removed to form a denture negative model. Then, a dough-form mixture of a powdery component composed mainly of a methyl methacrylate polymer and a liquid component composed mainly of a methyl methacrylate monomer is filled and pressurized in that model. After heating in hot water of about 70.degree. C. for 30 to 90 minutes, the flask is heated in boiling water for further 30 minutes. The ordinary heat curing method thus requires a time of as long as 60 to 120 minutes, and is therefore inefficient.
Recently, the technique for polymerization making use of microwaves has been developed to achieve such heat curing within a short period of time (see "The Journal of Dental Technology", Vol. 12, No. 8, 1984.8., pp. 965-974), and has begun to be used. According to this technique, a previously prepared wax denture is invested in plaster with the use of FRP flask. After the plastic is set, the wax is removed to obtain a denture negative model formed of plaster. Filled and pressurized in that negative model is a dough-form mixture comprising an ordinary heat-cured type resin for denture base (hereinafter referred to as the heat-curing base resin) consisting of a powdery component composed mainly of a methyl methacrylate polymer and a liquid component composed mainly of a methyl methacrylate monomer. The flask is then placed in a microwave oven for domestic use, wherein it is irradiated with microwaves for 3 to 5 minutes for polymerization.
However, the following problems arise, when it is intended to polymerize a denture by the microwave curing method, using the ordinary heat curing base resin.
(1) Short-time irradiation of microwaves does not allow sufficient polymerization, leaving unpolymerized matters which leads to formation of an inhomogeneouly set product.
(2) Porosity occur in the interior of the resin of a thick portion of the denture.
(3) When a denture including a clasp or a bar or a metal-plated denture is polymerized, porosity occurs in the interior of the resin around the metal.
(4) The polymerized denture deforms with the lapse of time, thus leading to a drop in fitting accuracy.
When the denture containing such problems are clinically used in actual applications, the residual monomer gives a stimulus to the oral mucosa. Further, the denture gives an uncomfortable feeling during mastication due to unsatisfactory fitting caused by their deformation with time and may not fulfill its masticatory function. The presence of internal porosity offers will not cause serious problems that may affect their appearance. However, when an ordinary denture is polymerized, porosity intensively occur in the maximum-occlusal-stress-applied interior of the resin around the alveolar below the base plane of the artificial teeth, which has a grave influence upon the durability of the denture.
When a denture including a clasp or a bar or a metal-plated denture is polymerized, porosity occur in the interior of the resin around the metal. In this case, a further considerable influence is produced upon the durability of that denture, since concentrated stress is applied on the portion where the metal is embedded in the resin, particularly the portion where the metal is maintained by the resin. Thus, when the ordinary heat-curing base resin is polymerized with microwaves, it seems that the denture is polymerized within a short time. As mentioned above, however, the actually obtained denture offers certain problems in view of practical use, and is unsatisfactory. For that reason, there are demanded resinous compositions for denture base having improved performance and, in particular, suitable for use with the microwave curing method.